Optically-generated dynamic nuclear polarization is increased in viscous mixtures due to longer lived electron spin polarization.

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced, using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism (RTPM). Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results, by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements, by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of pulsed laser rather than continuous-wave illumination, rationalising the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of illumination duty cycle the conditions necessary to permit use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology. I. INTRODUCTION Among all of the hyperpolarization techniques for overcoming the intrinsically low sensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy, Dynamic Nuclear Polarization (DNP) is the most studied and widely applicable. 1 It was first proposed by Overhauser in 1953, 2 and demonstrated experimentally by Carver and Slichter, 3 that saturating Electron Paramagnetic Resonance (EPR) transitions results in an increase in NMR sensitivity. This effect, due to cross-relaxation between the electron and nuclear spins, was later extended from lithium to free radicals in solution, 4 but has never been widely applied due to technical challenges and problems with reducing Overhauser efficiency at the high magnetic fields necessary for high resolution NMR spectroscopy.

Magnetic resonance methods are widely used to provide atomic level information on the structure and dynamics of chemical and biochemical systems, but often suffer from poor sensitivity. This review examines how optical excitation can provide increased electron spin-polarization, and how this can be used to increase sensitivity and/or information content in both Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopy.

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Magic angle spinning nuclear magnetic resonance spectroscopy experiments are widely employed in the characterization of solid media. The approach is incredibly versatile but deleteriously suffers from low sensitivity, which may be alleviated by adopting dynamic nuclear polarization methods, resulting in large signal enhancements. Paramagnetic metal ions such as Gd 3+ have recently shown promising results as polarizing agents for 1 H, 13 C, and 15 N nuclear spins. We demonstrate that the widely available and inexpensive chemical agent Gd(NO 3 ) 3 achieves significant signal enhancements for the 13 C and 15 N nuclear sites of [2- 13 C, 15 N]glycine at 9.4 T and ∼105 K. Analysis of the signal enhancement profiles at two magnetic fields, in conjunction with electron paramagnetic resonance data, reveals the solid effect to be the dominant signal enhancement mechanism. The signal amplification obtained paves the way for efficient dynamic nuclear polarization without the need for challenging synthesis of Gd 3+ polarizing agents.